I want to encrypt long paragraphs of text using Rijndael-256 (the text has been compressed and converted to base64 before this).

It is very unlikely for the two to be the same. But if they happen to be, would adding a random key to the start or end of the text secure it (regardless of whether they're the same), just in case users write the same text?

If I make sure that no results are 100% the same, is ECB safe if you won't get duplicate results?